Sweet sorghum, Sorghum bicolor (L.) Moench is the only crop that provides grain and stem that may be used for the production of alcohol, sugar, syrup, fuel etc. But the major problem with this plant is the presence of lignin in cell wall that adversely affects the process of extraction of beneficial materials. Alteration of lignin content is likely to improve the quality of the plant. All conventional breeding programs undertaken to generate cultivars with reduced lignin content met with limited success. Thus, possibility to develop cultivars with altered lignin content through genetic engineering figured as a distinct possibility.
Lignin is considered to be dehydrogenatively polymerized from the monolignols p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. These monolignols are synthesized through the phenylpropanoid pathway. Structurally these monolignols differ only by the methoxy group at the 3C and 5C positions of the aromatic ring. Varying proportion of the monolignols determine the type of lignin. Variation/heterogeneity in lignin molecules depend on the amount of a particular component: when p-coumaryl alcohol is present in higher amount over other components, it is called hydroxyphenyl (H) lignin; when coniferyl alcohol is present in higher amount over other, it is called guaiacyl lignin (G). Similarly, if the content of sinapyl alcohol exceeds the other components, then it is called syringyl lignin (S). G lignin offers more resistance than S during enzymatic degradation. G lignins are more condensed due to more numbers of intermolecular linkages, thereby showing more resistance. It was observed that lignin levels increase with progressive maturity in stems of forage crops, including legumes such as alfalfa (Jung et al., 1997) and in grasses such as tall fescue (Buxton and Redfearn, 1997). Moreover, the lignin composition changes with advanced maturity toward a progressively higher S/G ratio (Buxton and Russell, 1988).
Several approaches have been taken to decrease lignin content and to increase S/G ratio. However, the results have found to be contradictory, possibly due to lack of understanding of lignin biosynthetic pathway and due to inappropriate suitable approaches for down regulation of the lignin biosynthetic enzyme activity including choice of transgene, promoter used, construction of antisense cassettes and above all, selection of transformants. Regulation of early steps enzymes like phenylalanine ammonia lyase, cinnamate 4-hydroxylase, 4-hydroxycinnamate CoA ligase reduced lignin content. However, it leads to pleiotropic effects including altered leaf shape, localised fluorescent lesion, stunted growth, reduced pollen activity, altered flower morphology and pigmentation, reduced level of soluble phenylpropanoids, decrease in S/G ratio etc (Elkind et al, 1990; Bate et al, 1994; Sewalt et al, 1997). Similar effects by other workers to alter or modify the S/G ratio have resulted in phenotypically defective plants. It was demonstrated that down regulation of caffeic acid O-methyltransferase activity could result dramatic decrease in syringyl lignin biosynthesis but with little effect on the synthesis of guaiacyl lignin, which is undesirable as the latter are more resistant to chemical degradation. In this background the present invention provides a novel transgenic sweet sorghum plant having modified lignin content in cell walls using a different approach in construction of the antisense gene cassettes.